A. Field of the Invention
This invention relates to the field of data communication and data-over-cable systems. More particularly, the invention relates to a method of changing the channel at which a cable modem transmits in the upstream direction to a cable modem termination system.
B. Description of Related Art
With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems, such as those provided by 3Com Corporation of Santa Clara, Calif., and others offer customers higher-speed connectivity to the Internet, an intranet, local area networks (xe2x80x9cLANsxe2x80x9d), and other computer networks via cable television networks. These cable modems currently support a data connection to the Internet and other computer networks via a cable television network with a data rate of up to 30+ Mbps, which is a much larger data rate than can be supported by a modem used over a standard telephone line. Background information related to cable modem systems in general is described in the Data-Over-Cable Service Interface Specifications (xe2x80x9cDOCSISxe2x80x9d)xe2x80x94Radio Frequency Interface Specifications, Interim Draft, dated Nov. 5, 1999, issued by Cable Television Laboratories, Inc. This document, known to persons working in the art, is incorporated by reference herein in its entirety.
Data-over-cable systems may serve many cable modems simultaneously. A cable modem receives and transmits data in the form of digitally encoded radio frequency transmissions, which typically travel to and from the cable modem on, for example, coaxial cable. Also, the data-over-cable system consists of many branches from a common trunk, each branch serving a distinct group of cable modems, and the branches themselves may further divide into sub-branches. Thus the propagation of radio frequency signals to and from the cable modem often follows a circuitous and convoluted path. Transmissions from the cable modem are terminated in a cable system head-end by a device known as a Cable Modem Termination System or CMTS.
Sometimes, one or more cable modems develops a fault. Because return signals from the cable modems arrive from their respective branches, it is often difficult to isolate faults in the network by examining combined return signals on the common trunk. Moreover, a fault in the coaxial cable serving a branch of the network may impair the signals for all the cable modems served by that branch. Thus a faulty cable modem""s signals, or signals from a faulty branch of the network, may corrupt the signals of other cable modems on the common trunk. Such corruption frequently defeats attempts to distinguish and identify which cable modems or branches require repair.
Information concerning the location and nature of an impairment may be very useful to a cable service provider. Such information would allow the provider to map cable modems to faults in the data-over-cable network, thus enhancing the cable service provider""s ability to diagnose and isolate problems in the data-over-cable network, and thereby reducing repair times and maintenance costs. A radio-frequency performance monitor is described in the patent application of Mark O. Vogel, entitled xe2x80x9cRadio Frequency Performance Monitor for a Data-Over-Cable Systemxe2x80x9d filed Aug. 9, 2000, Serial No. 09/634,780, the contents of which are incorporated by reference herein.
When impairments in the upstream channel from the cable modem to a CMTS exist, cable modem systems provide for the ability to change the upstream channel that the cable modem transmits in, that is, in the direction between the cable modem and the CMTS. In the prior art, the only way to move a registered modem from its current upstream channel to a new upstream channel is for the CMTS to issue an Upstream Channel Change request (UCC-REQ) message. When a modem receives this message, it must first issue an Upstream Channel Change Response message (UCC-RSP) on its current channel, and then it can move to the new channel as directed. However, to send a UCC-RSP message, a modem must actually first request an upstream opportunity for transmission of the UCC-RSP. To make the bandwidth request, a modem must wait for an Upstream Bandwidth Allocation Map message (MAP) with a Request (REQ) interval. During that interval, a modem can request the amount of bandwidth it needs to send its UCC-RSP message. Assuming that it does receive a REQ opportunity, and that the message is successfully received by the CMTS, the modem again must wait for the MAP that provides it with a grant for its requested upstream bandwidth. Upon receipt of this, the modem can then send its UCC-RSP message at the appropriate time defined by the MAP, and change the upstream channel.
A problem can occur with this prior art sequence. In particular, since the REQ interval is a contention interval, other modems may be competing for the same REQ opportunity. If another modem happens to make a request in the same interval, there will be a data collision and the CMTS will ignore the corrupted transmission. In this case, the modem will initiate its back off algorithm and attempt another bandwidth request transmission after waiting the number of REQ intervals specified by the algorithm. If the network is busy, this collision and back off process can occur multiple times, which can lead to a significant delay from a time a UCC-REQ is issued to the time the modem is actually able to move to the specified channel. In fact, after 2 seconds the CMTS will issue another UCC-REQ message to the modem.
Because the time associated with the UCC message exchange is not deterministic, that is, it cannot be known in advance, this method of operation is inadequate for voice applications, such as Voice over Internet Protocol (VoIP) applications, Internet telephony, Internet video on demand, or other time critical services, where service can be lost by such delays or degraded low service quality objectives. In addition, because of the potentially long time associated with the UCC message exchange, dynamic load balancing becomes inefficient. To avoid these problems, a new way of initiating a channel change is needed in the art which is (1) more deterministic, (2) which requires much less time to complete than the standard UCC message exchange, and (3) that allows dynamic load balancing to be efficiently achieved.
Whereas the prior art has failed to adequately address the upstream channel change problems in data-over-cable systems, the present invention provides an upstream channel change mechanism which is deterministic, can be completed in a sufficiently short period of time such that time critical services can be provided, and dynamic load balancing objectives can be efficiently reached.
A method of changing an upstream channel for a modem in a data-over-cable system is provided. The system includes a cable modem termination system (or the equivalent) that generates upstream bandwidth allocation map (MAP) messages for cable modems in the system. In accordance with the method, the cable modems monitor the MAP messages associated with an alternate upstream channel in the data-over-cable system for an indication that the cable modem may change the channel it currently transmitting in the upstream direction to a new channel. In response to the monitoring, the modem moves from its current channel to the new channel. Accordingly, since the method does not require exchange of upstream channel change request messages and associated reply messages, and the concomitant delays, the change in upstream channel may be achieved in an efficient manner without substantial delay. Furthermore, the dynamic load balancing in the upstream direction can be achieved efficiently.
In the illustrated embodiment, the cable modem monitors MAP messages associated with its current channel and MAP messages associated with other channels in the data-over-cable system that are viable alternate channels to the current channel. Basically, the cable modem is looking for an indication in a MAP message in the alternate channel, in which the indication comprises a unicast upstream transmission opportunity for the modem. The cable modem moves from the current channel to the new channel upon receipt of a MAP message in the alternate channel containing the upstream transmission opportunity. Several types of unicast upstream transmission opportunities are described.
In another aspect of the invention, a cable modem is provided which implements the method as described above, namely the monitoring of MAP messages and responsively changing its upstream channel.
In still another aspect, an improvement to a data-over-cable system is provided. The improvement comprises a cable modem termination system that transmits upstream bandwidth allocation map (MAP) messages for monitoring by cable modems in a data over cable system. The MAP messages contain fields with data instructing cable modems to change the upstream channel the cable modems use for transmitting data to the cable modem termination system. An upstream channel change is effected by the cable modems monitoring the MAP messages. The invention does not require an Upstream Channel Change Request message to be sent from said cable modem termination system to the cable modem, or a reply to such as request message, thereby overcoming the problems associated with prior art methods for changing upstream channels in such data-over-cable systems.